Polyester and polyamide adhesion improvers

ABSTRACT

Prior to applying an alkyd paint to a metallic surface or to a pre-paint coating covering a metallic surface, there is applied to said surface or said coating a polyester or a polyamide for the purpose of improving the adhesion of said paint. The polyester or polyamide can be applied from an aqueous rinse solution or from a final rinse solution.

[54] POLYESTER AND POLYAMIDE ADHESION 3,625,777 12/1971 Okabe et a]148/615 Z IMPROVERS 1 2,636,257 4 1953 Ford 117 75 x 2,887,405 5/1959Wooding 117/75 X Inventors: Thflmas .1- Kiefer, Ambler, Pa; 3,164,4881/1965 Workman 117 75 Masamiclii Nagatani, Tokyo, Japan 3,565,699 2/1971Plaxton 148/616 [73] Assignee: Amchem Products, Inc., Ambler, Pa.3573997 4/1971 Plaxtonw 148/616 [22] Filed: Mar. 15, 1971 PrimaryExaminer-Ralph S. Kendall [211 App No 124 562 Att0rney-Synnestvedt &Lechner [52] US. Cl. 148/615 Z, 148/6.15 R, 148/616, [57] ABSTRACT 11775 Prior to applying an alkyd paint to a metallic surface 51 1111. cu.@231 7/08 or to a P -p coating covering a metallic Surface, [58] Field61 Search 260/2335; there is pp to Said Surface or Said coating a p y-148/6.15 R, 6.15 Z, 6.16; 117/84, 165, 75 ester or a polyamide for thepurpose of improving the adhesion of said paint. The polyester orpolyamide can [56] References Cited be applied from an aqueous rinsesolution or from a UNITED STATES PATENTS final rinse 501mm 3,519,4957/1970 Plaxton 148/616 11 1 11 N0 Drawings POLYESTER AND POLYAMIDEADHESION IMPROVERS FIELD OF THE INVENTION This invention relates to thetreatment of metallic surfacesfor the purpose of improvingthe adhesionof a resinous coating applied thereto. More particularly, this inventionrelates to improving the adhesion of an ester-type resinous coating,such as an alkyd resinous paint, to a metallic surface that has thereona pre-paint coating.

It is well known to coat metallic surfaces prior toapplying thereto aresinous coating or paint film which is aesthetic and/or functional innature. A function of the pretreatment coating, often referred to asa'pre-paint coating, is to inhibit corrosion of the metallic surfacethat would tend to be caused by moisture or other corrosive materialspenetrating the resinous coating or paint film. Also, the pre-paintcoating should inhibit or prevent undercutting of the resinous coatingor paint film that would tend to be caused by corrosive materials actingon the metallic surface as a result of the organic film being chipped,scratched, pierced or otherwise ruptured.

An example of a pre-paint coating which forms a corrosion resistantbarrier on a ferriferous surface is a zinc phosphate coating. Corrosionresistant chromate coatings are examples of coatings which protectaluminum surfaces from corrosive materials.

To function as intended, the pre-paint coating must 'be such that theresinous coating or paint film adheres tightly thereto.

It is noted that many types of metallic surfaces are relatively poorpaint bases, that is, the resinous coating or paint film does not adherewell to the bare metallic surface. In some applications, the primarypurpose of the pre-paint coating is to provide a base to which theresinous coating or paint film will adhere readily and tightly; anyprotection against corrosion that is afforded by the pre-paint coatingis of secondary importance.

A popularly used film-forming resin that is applied to metallic surfaceshaving thereon a pre-paint coating is an alkyd resin. An alkyd resin isa polyester that can be prepared by reacting a polyhydric alcohol, apolybasic acid and a monobasic fatty acid.

This invention relates to improving the adhesion between a film-formingpolyester resin, for example, an alkyd resin and a metallic surface,particularly a metallic surface that has thereon a pre-paint coating.

PRIOR DEVELOPMENTS The relatively few developments in the prepaint ormetal pre-treatment field directed to improving paint adhesion have oneor more shortcomings.

A method for improving paint adhesion, but a limited one in that it isconfined to zinc phosphate coatings, is to produce zinc phosphatecoatings with lower coating weights. The general rule is that the lowerthe coating weight of a zinc phosphate coating the better the paintadhesion. One way of lowering the coating weight is to modify a zincphosphate coating solution in a manner such that the zinc phosphatecoating formed from the solution has a lower weight than would beobtained had the coating solution not been modified. Additives whichhave been incorporated into zinc phosphate coating solutions to producelower coating weights are disodium glycerol phosphate and starchphosphate. Another way of producing a lower weight coating is to subjectthe zinc phosphate coating to an after treatment which dissolves some ofthe coating. This can be done by contacting the coating with a strongacidic solution.

A disadvantage of the above method is that the improvement in paintadhesion is obtained usually at the expense of some loss in corrosionresistant properties of the coated metallic surface.

Improvements in paint adhesion have been obtained also as a result oftreating pre-paint coatings with final rinse solutions. For example, ithas been reported that the rinsing of aniron phosphate coating with adilute aqueous rinse solution of chromic acid can result in improvedadherence of the paint film to the coating. In U.S. Pat. No. 3,063,877,it is reported that a final rinse solution prepared from the reaction ofhexavalent chromium and formaldehyde and containing both hexavalent andreduced chromium is effective in improving both the corrosion resistantand paint adhesion properties of pre-painted and painted metallicsurfaces.

Although final rinse solutions such as those mentioned above can beeffective in improving paint adhesion, it is recognized generally thattheir principal value is in improving the corrosion resistant propertiesof the coated surface either by passivating exposed metal or by sealingthe pores of the pre-paint coating. Except perhaps in some limitedcases, the extent to which they improve paint adhesion tends to bemarginal.

In the overall picture, there is a need for improving the adhesion ofpaint to metallic surfaces and it is an object of this invention toprovide the means to do so.

BRIEF SUMMARY OF THE INVENTION In accordance with this invention it hasbeen found that improvements in paint adhesion can be realized byapplying to a metallic surface or to a metallic surface having thereon apre-paint coating, an adhesion improver, as described below, andthereafter applying to the thus treated surface a film-forming resinouscoating composition comprising a polyester resin, for example an alkydresin. Adhesion improvers within the scope of this invention includepolyesters and polyamides.

It is believed that the invention will have its widest use inapplications wherein the adhesion improver is applied from an aqueoussolution to a pre-paint coating formed on a metallic surface. Theadhesion improver can be dissolved or dispersed in a water rinse whichis used to rinse the pre-paint coating to remove therefrom excess orspent pre-paint coating solution. Or it can be dissolved or dispersed ina final rinse solution which is applied to the pre-paint coating toimprove its corrosion resistant properties.

As will be explained more fully below, the adhesion improver can bedeposited on the metallic surface or on the prepaint coating in the formof discrete particles or as a continuous or discontinuous film.Generally speaking, the thickness of the film of adhesion improver willbe substantially less than the coating or film that is formed from thesubsequently applied filmforrning resinous coating composition. Forexample, the thickness of the film of adhesion improver can be on theorder of several molecules in thickness or even less.

After the adhesion improver has been deposited, the surface can becovered with a film-forming resinous coating composition comprising apolyester resin. The term film-forming resinous coating composition asused herein is intended to include both pigmented and non-pigmentedcoating compositions and thus encompasses within its meaning paints andalso clear resinous coatings such as, for example, varnishes andlacquers. It is believed that the invention will have its widest usewith alkyd paints a very popular polyester paint, as noted above.

The presence of the adhesion improver of this invention on the surfaceimproves the bond between the coating formed from the film-formingresinous coating composition and the surface. This, of course, is animportant advantage because coating or paint loss can be lessened oravoided when the metal is deformed as by bending, impact or formingoperations.

Another advantage that is provided by this invention is that improvedpaint adhesion properties can be realized absent an adverse affect oncorrosion resistant properties. Furthermore, the adhesion improvers ofthe present invention can be applied in prepaint treatment stages thatpresently are used in commercial prepaint treatment lines.

DETAILED DESCRIPTION OF THE INVENTION The adhesion improvers of thisinvention are used with film-forming resinous coating compositions whichmust contain a polyester resin. Polyester resins are polymericcondensation products having a backbone in which hydrocarbon units whichmake up the backbone are joined together by ester linkages Polyesterresins can be prepared by the esterification between a polycarboxylicacid and a polyhydric alcohol or by polymerizing a hydroxy-substitutedcarboxylic acid. Many types of polyester resins can be used infilmforming resinous coating compositions.

In work connected with the development of the present invention, it wasfound that improvements in paint adhesion are not attained when thefilm-forming resinous coating composition does not contain a polyesterresin, but instead contains other types of resins having ester linkageswhich are not part of the backbone of the resin. Examples of such resinsare the acrylics and the acrylates in which the ester linkages are partof the side chains of the polymer but not the backbone thereof. However,and as will be discussed more fully below, this invention can bepracticed effectively with filmforming resinous coating compositionswhich contain such polymers as the acrylates and acrylics, as well asothers, as long as the film-forming resinous coating compositioncontains a polyester resin also.

The most widely used polyester resins in film-forming resinous coatingcompositions are resins prepared by esterifying a polyhydric alcohol, apolybasic acid and a monobasic fatty acid. Such resins are termed alkydresins. Authoritative discussions on alkyd resins and polyester resinsare found in the following publications: The Chemistry of SyntheticResins by Ellis, Volume II, Reinhold Publishing Corporation, New York,1935, and The Encyclopedia of Chemical Technology by Kirk and Othmer,Interscience Publishers, New York, l963.

Examples of polybasic acids (or anhydrides thereof) which are used inpreparing alkyd resins are: phthalic anhydride; azelaic acid; maleicanhydride; and adipic acid. Examples of polyhydric alcohols that areused in the preparation of alkyd resins are: glycerol; pentaerythritol;and propylene glycol. Examples of monobasic fatty acids and sources ofmonobasic fatty acids which are used in preparing alkyd resins arepelargonic acid; isooctanoic acid; linseed oil; cottonseed oil; and tungoil. Exemplary number average molecular weights of alkyd resins preparedfrom such reactants are about 2,000 to about 3,000.

There are a wide variety of alkyd resins available which can be used inthe practice of this invention. For example, the alkyd resin may be ofthe drying or nondrying type. Alkyd resins, the oil content of whichvaries widely or which contain varying amounts of phthalic anhydride,can be used also.

Alkyd resins of the type termed generally as modified alkyds can be usedalso. Modified alkyd resins are prepared by including in theesterification a reactant which imparts one or more desired propertiesto the alkyd resin produced. Examples of such reactants or modifiersare: isocyanates; silicones; phenolics; epoxides; and rosin.

The polyester resin in the film-forming resinous coating composition cancomprise a major or minor proportion of said coating composition. Inthis connection, it is noted that many alkyd resins are compatible withother film-forming polymeric materials and when combined therewith, thealkyd resins impart desired properties to the resulting composition orto the film or coating that is formed. Durability and coatingflexibility are examples of such properties. Examples of such otherfilm-forming polymeric materials are vinyls, epoxies, acrylics,styrene/butadiene and phenol/formaldehyde resins. Resinous coatingcompositions containing but a minor proportion of an alkyd resin forexample, at least about 2 wt. percent can be used in the practice ofthis invention.

On the other hand, the polyester resin can constitute the principalfilm-forming resin or resinous ingredient in the coating composition.Speaking generally, such compositions can comprise about 20 to about 40wt. percent of the polyester resin. Other materials are often blendedinto such coating compositions to impart beneficial properties to thecomposition or the coating formed therefrom. Examples of such materialsare nitrocellulose, chlorinated rubber and epoxies.

The film-forming resinous coating composition containing the polyesterresin can be pigmented or nonpigmented. For the purpose of convenience,the term alkyd paint when used herein includes within its meaning bothpigmented and non-pigmented filmforming resinous coating compositions inwhich the principal film-forming resin or resinous ingredient is analkyd resin.

The film or coating formed from the polyestercontaining, film-formingresinous coating composition can be an undercoating or a final finishcomprising a continuous polymeric film which is preferably free of voidsand uniform in thickness. Many of the available film-forming resinouscoating compositions need to be baked after application in order to beformed into a continuous film. Although in some applications, saidresinous coating composition can be air dried, it will be more practicalmost of the time to bake said composition in order to form it into thecontinuous film. The continuous film or coating formed from thepolyestercontaining, film-forming resinous coating composition will besubstantially greater in thickness than an underlying film of adhesionimprover.

As mentioned above, adhesion improvers included within the scope of thisinvention are polyesters and polyamides.

The polyesters can be formed by the condensation reaction between analcohol and either an organic acid or an inorganic acid or theirderivatives such as anhydrides and acyl halides; they can be preparedalso by polymerizing a hydroxyl-containing carboxylic acid. in preparingthe polyester adhesion improver, the reactants should be selected sothat relatively long chain molecules having many ester linkages will beproduced. This can be accomplished by polymerizing a polyhydric alcoholwith a polybasic acid. In general, best improvements in paint adhesionproperties have been attained when both reactants are poly-functional innature, that is, the alcohol is a polyol and the acid has two or morecarboxyl groups.

The polyester adhesion improvers, which can contain aliphatic oraromatic groups, can be saturated or unsat urated. Exemplary reactantsthat can be used to prepare the polyesters are dihydric and trihydricalcohols, as well as higher numbered hydroxyl-containing alcohols,including polymers having a substantial number of hydroxyl groups.Similarly, dibasic and tri-basic acids, as well as higher numberedcarboxyl-containing acids, including polymers which contain asubstantial number of carboxyl groups, can be used in preparing theadhesion improvers of this invention. When an inorganic acid is the acidreactant in the esterification, it should be reacted with a polyol.

Examples of organic acids and anhydrides that can be used to prepare thepolyester adhesion improvers include lauric acid, glutaric acid,decane-dicarboxylic acid, isophthalic acid, hexahydrophthalic anhydride,itaconic acid, citric acid and polymers containing a substantial numberof carboxyl groups such as polymethylvinylether maleic acid and carboxymethyl cellulose.

Examples of inorganic acids that can be reacted with alcohols to preparepolyester adhesion improvers within the scope of this invention includephosphoric acid, boric acid, and sulfuric acid.

Examples of alcohols that can be used to prepare the polyester adhesionimprovers include propylene glycol, butylene glycol, erythritol, andpolymers containing a substantial number of hydroxyl groups such aspolyvinyl alcohol.

Other materials not referred to commonly as alcohols or acids, but whichcontain hydroxyl or carboxyl groups can be used also to prepare thepolyester adhesion improvers. Examples of such materials include Istarch and cellulose.

Moisture, 4.5 Protein, 0.3 Ash, 7.l Sodium (calculated), 1.8 pH 7.1Solubility Cold water soluble Viscosity, Brooktield No. 4 spindle at 6RPM, 5% aqueous solution, as is, at C, cps

20,000 Color of dry powder Light tan Gel strength N0 gel informationwhen paste from viscosity determination is aged 24 hrs. at 25C.

Starch phosphates, such as the one described above, can be prepared byimpregnating or blending starch granules with an aqueous solution of analkali metal phosphate, removing excess moisture and thereafter dryroasting the resultant product to esterify the starch (see US Pat. No.2,884,412). Starch phosphates so produced can contain about 1 to about 5percent by weight of bound phosphorous based on the weight of the drystarch. The starch phosphates in water form clear dispersions which arebelieved to be colloidal in nature, but which, for convenience, arereferred to usually as solutions.

' Starch phosphates can be prepared by other methods also. For example,they can be prepared by reacting starch and phosphorous oxychloride inthe presence of a hydrogen chloride acceptor such as, for example,pyridine. In addition, starch phosphates can be prepared by the reactionof orthophosphoric acid and corn starch.

Speaking generally, the amount of bound phosphorous in the starchphosphate comprises a minor portion of the starch molecule. For examplethe starch phosphate can contain about 0.1 percent to about 8.0 percentby weight of bound phosphorous.

As to the polyamides and their use as adhesion improvers in thepractice-of this invention, they can be prepared according to anyavailable method. For example, they can be prepared by tye condensationreaction between a polycarboxylic acid and a polyamine; or by reacting apolycarboxylic acid with a hydroxy amine; or by the polycondensation ofamino acids. As with the polyester adhesion improver, the reactants forpreparing the polyamides should be selected so that relatively longchain molecules having many amide linkages will be produced.

Exemplary organic acids that can be used to prepare the polyamideadhesion improvers are sebacic, succinic, azelaic and oxalic acid.

Examples of amines that can be used to prepare the polyamide adhesionimprovers include diethanolamine, hexamethylenediarnine andpropylenediamine.

The following are examples of amino acids that can be polymerized toprepare polyamide adhesion improvers: glycine, alanine and4-aminobutyric acid.

The reactions by which the polyester and polyamide adhesion improversare made are well known, of course, and thus they warrant no descriptionherein.

However, in order to obtain the operating advantages of the presentinvention to the fullest extent, it is recommended that the reactantsand reaction conditions be selected so that the adhesion improverproduced is a long chain molecule which is water soluble or colloidallydispersible. This allows the adhesion improver to be deposited on themetallic surface or the pre-paint coated metallic surface from anaqueous solution. (The term solution" as used herein includes truesolutions and colloidal dispersions.) This is a great practicaladvantage.

It is believed to be impractical, if not impossible to describe thepolyester and polyamide adhesion improvers by molecular weight,viscosity, etc. because of the many different types of reactants thatcan be used and the very different types of polyesters and polyamidesthat are operable. However, with respect to the preferred mode ofpracticing the invention, that is, by utilizing aqueous solutions of thepolyester or polyamide, it is recommended that the reactions by whichthey are made be allowed to continue until the reaction productsincrease in molecular weight to the greatest extent that is compatiblewith their being water soluble or colloidally dispersible.

As mentioned briefly above, the adhesion improver can be present on themetallic surface or pre-paint coating in the form of a continuous ordiscontinuous film or as discrete particles. A discontinuous filmappears as blotches of film on the surface or coating. A continuous ordiscontinuous film tends to be formed when the adhesion improver isapplied from a true solution. Such films can be monomolecular inthickness or a few or several molecules thick. On the other hand, whenthe adhesion improver is applied from a colloidal solution or from asuspension it can be present on the surface in the form of discreteparticles.

It is believed that the improved paint adhesion properties of thisinvention are attained as a result of a reaction between the adhesionimprover and the polyester resinous ingredient in the film-formingresinous coating composition. When the adhesion improver is a polyester,the reaction can include an ester exchange, referred to also astransesterification, that is, a hydrocarbon group of the polyester paintadhesion improver links to the of an ester group in the polyester resinof the overlying resinous film or coating as a hydrocarbon group fromthat polyester resin links with the 0 of an ester group in theunderlying polyester adhesion improver. The structural formulas setforth below illustrate this in a very simplified way with the topstructural formula representing a polyester resin in the resinous filmor coating and the bottom formula representing a polyester adhesionimprover.

O O O O hatmnto atoBartok] R"-C odmn cort" t t t 1 The bonds above are asimplified illustration of how the different polyesters can linktogether to thereby become integral.

With respect to the use of a polyamide adhesion improver, a similar typeof interchange can take place between the hydrocarbon groups, exceptthat the groups link to the -N atom of the group.

Thus, the present invention provides a method for improving the adhesionbetween a metallic surface or a pre-paint coating carried on a metallicsurface and a coating formed from a film-forming resinous coatingcomposition by applying to said surface or said prepaint coating anadhesion improver material which is capable of reacting chemically withsaid coating composition. Upon reacting, molecules of the resin in thecontinuous polymeric film that is formed from the coating compositionand molecules of the adhesion improver become linked. it can be seenfrom the illustration above that molecules of each of the adhesionimprover and the resin of the coating composition have many reactablesites and that atoms of the one molecule are bonded to atoms of theother molecule. In those situations where heat is applied to the coatedmetallic surface to accelerate the formation of the continuous polymericfilm, the heat can function also to accelerate the chemical reactionbetween the adhesion improver and the coating composition. It should beunderstood, however, that the adhesion improver can react with theresinous coating composition in the absence of heat.

In the development of this invention, it has been found that thetreatment of a bare metallic surface, that is, one which carries nopre-paint coating, with the adhesion improvers of this inventionimproves the bond between the polyester film-forming resinous coatingcomposition applied subsequently and the thus treated surface.Improvements in the paint bond can be obtained also if the adhesionimprover is applied to a metallic surface which carries no pre-paintcoating, but is one which has been pretreated in some other way--forexample pre-treated to activate the metal. For most applications, theadhesion improvers will be applied to a metallic surface that hasthereon a pre-paint coating which has corrosion resistance properties.

Any suitable pre-paint coating can be used. It can be of the type whichis bonded chemically to the metallic surface or it may be bondedphysically thereto. The pre-paint coating may be either organic orinorganic. It can be crystalline or amorphous in nature. The coating canbe applied by an electrolytic process or in the absence of an appliedcurrent. The coatings can be formed from ingredients in the coatingcomposition and they can include also amounts of the metal being coated;and they can be formed also from ingredients which are the product of areaction between the ingredients of the coating composition, whichreaction may or may not include ions of the metallic surface.

The term pre-paint coating when used herein means coatings for metallicsurfaces which coatings are bonded chemically or physically to themetallic surface and which coatings have corrosion resistant and/orpaint adhesion properties and which coatings underlie a film-formingresinous coating composition applied thereto.

Representative types of pre-paint coatings and the coating compositionsor solutions from which they are formed are discussed in Preparation ofMetals for 9 Painting by Samuel Spring, Ph.D., Reinhold PublishingCorporation, New York, 1965.

Generally speaking, pre-paint coatings are formed on metallic surfacesfrom coating solutions, most often aqueous solutions, which are appliedto the metallic surface in any one of a variety of ways, including, forexample, spraying, rolling, brushing or immersing the metallic surfacein a bath of the solution. For exemplary purposes, there are describedgenerally in the next few paragraphs some of the more popularly usedpre-paint coatings; the description includes exemplary types ofcoatings, the metals on which they are deposited and a generaldescription of the coating compositions from which they are formed. Itwill be recognized that such coating compositions form coatings whichare often referred to as conversion coatings."

Zinc phosphate coatings are corrosion resistant, paint adherent coatingswhich can be applied to such metallic surfaces as, for example,ferriferous (ironcontaining, including steel) aluminum, cadmium, andzinc (including galvanized surfaces). Zinc phosphate coatings can beformed from aqueous coating solutions prepared from zinc oxide andphosphoric acid, and the solution contains usually an oxidant such as,for example, chlorate, nitrate, or nitrite.

Acidic aqueous solutions prepared from an alkali metal phosphate areused to form iron phosphate coatings on ferriferous surfaces.

Alkaline chromate solutions prepared from alkali metal chromates ordichromates can be utilized to form paint adherent coatings on aluminum.

Acidic aqueous coating solutions containing fluoride, phosphate anddichromate are utilized to form chromate/phosphate coatings on aluminum.

Also, chromate/fluoride coatings formed from acidic aqueous solutions ofchromate and fluoride are used to inpart corrosion resistant and paintadherent properties to aluminum.

. Other examples of pre-paint coatings are oxalate coatings forferriferous surfaces, mixed oxide and chromate coatings formed fromaqueous solutions of chromic acid for ferriferous surfaces, chromatecoatings for zinc, and also such coatings for magnesium as are formedfrom aqueous coating solutions of dichromate and aqueous solutions offerric nitrate. And still other examples of pre-paint coatings that canbe used in the practice of this invention are: organic coatings such as,for example, those formed from an aqueous solution containing a volatilealkali, a film-forming resin and hexavalent chromium as described inU.S. Pat. No. 2,921,858 and complex oxide coatings formed from alkalinebaths of selected sequestered metal ions.

The adhesion improvers of this invention can be applied to the bare orpre-paint coated metallic surface by any suitable method which willleave the improver on the surface. They can be applied at roomtemperature or elevated temperature. From an economical standpoint, itis most practical to apply the adhesion improvers from an aqueouscarrier. For example, after the application of a pre-paint coatingsolution to a metallic surface, it is rinsed with water usually for thepurpose of removing spent or excess coating solution from the surface.The adhesion improvers of this invention can be incorporated in therinse water. Or the adhesion improver can be incorporated in an aqueousfinal rinse composition. It is common practice to rinse pre-paintcoatings with a final rinse composition-which is effective in improvingcorrosion resistant properties and which may improve paint adhesionproperties also. Presently, the most widely used final rinsecompositions are aqueous solutions which contain chromium in one form oranother. A typical chromium-containing final rinse is an aqueoussolution which can contain from about 0.002 percent to about 0.01percent by weight of hexavalent chromium.

In the practice of this invention, very good results have been obtainedby applying the adhesion improver from a chromium-containing final rinseof the type that is disclosed in US. Pat. No. 3,063,877. The aqueousrinse solution disclosed therein is prepared from a concentrated aqueousacid solution of chromic acid which solution has been treated withformaldehyde to reduce a portion of the hexavalent chromium. Morespecifically, the rinse solution can be prepared by reacting aconcentrated aqueous acidic solution containing from about 40 to about800 grams per liter of hexavalent chromium (expressed as CrO withformaldehyde to reduce about 5 to about wt. percent of the chromium andthereafter adding water to the concentrated solution in an amount suchthat the final rinse solution contains from about 0.15 to about 10 gramsper liter of total chromium. About 40 to about wt. percent of thechromium is in the hexavalent state with the remainder in the reducedstate. Such rinse solutions can be used for rinsing pre-paint coatingsapplied to zinc, ferriferous and aluminum surfaces to improve corrosionresistance. By incorporating the adhesion improver in such a final rinsesolution, it is possible to improve markedly paint adhesion propertiesalso.

in applications in which a final rinse is employed and in which theadhesion improver is applied from a water rinse, the metallic articleupon withdrawal from the water rinse can be placed directly into thefinal rinse without first drying the water-rinsed article. On the otherhand, if no final rinse is employed, then the metallic article should bedried after the adhesion improver is applied from the water rinse andbefore the filrn-forming resinous coating composition is applied.Similarly, when the adhesion improver is applied from a final rinse, theliquid carrier should be evaporated prior to the application of thetilm-forming resinous coating composition.

As mentioned above, after the application of the film-forming resinouscoating composition, it will be necessary usually to bake the coating'inorder to form a continuous film. On the other hand, it is not necessaryto bake the adhesion improver after it is applied to the metallicsurface or the pre-paint coating. Evaporation of the liquid carrierprior to painting is sufficient.

it should be understood that the adhesion improvers can be applied fromother types of final rinse compositions, includingnon-chromiumcontaining final rinse compositions.

As mentioned above, the preferred adhesion improvers are those which canbe dissolved in an aqueous carrier or those which will form an aqueouscolloidal solution. Although suspensions of the adhesion improvers canbe used, they generally will require agitation equipment to maintain theparticles suspended in the liquid carrier.

Although it appears that the adhesion improvers can be applied mostpractically and economically from a water carrier, they' can be appliedalso from a nonaqueous medium. For example, it will be readily apparentthat adhesion im'provers within the scope of this invention are solublein organic solvents such as, for example, butyl cellusolve, ethanol,butanol, xylene, and toluene.

Solutions containing the adhesion improver can be applied in anysuitable way. It should be apparent from what has been statedhereinabove, that the solution can be applied by existing equipment inindustrial metal pre-treatment lines for example, equipment whichapplies water rinses or final rinses to the pre-paint coatings.

It is not only difficult, but also impractical to give guidelines on theminimum and maximum amounts of adhesion improver that should bedeposited on the metallic surface or the pre-paint coating. Thedifficulty arises because of numerous variables that are inherent in theprocess, including both the pretreatment and painting steps of theprocess and also the specific adhesion improver employed. The variablescan influence the results that are obtained, that is, the deposition ofa specific amount of adhesion improver under one set of conditions maygive a different result under a differ ent set of conditions. Thefollowing are examples of variables that can have a bearing on theamount of adhesion improver to be deposited: the particular filmformingpolyester that is used in the film-forming resinous coating composition;the specific type of metal or pre-paint coating that is used; and thespecific adhesion improver that is applied.

From a practical standpoint it is believed that it is much easier toapply varying amounts of the adhesion improver until the desired resultis achieved than it is to analyze the surface for the amount of adhesionimprover thereon.

Notwithstanding the various factors that may have to be taken intoaccount in depositing appropriate amounts of the adhesion improver, itis suggested on the basis of experience with many different types ofpolyesters and polyamides that improvements in paint adhesion can berealized when the metallic surface is subjected to a solution whichcontains at least about 0.001 percent by weight of adhesion improver fora period of time as short as necessary to wet the surface and as long asabout 1 minute. The upper limit on the amount of adhesion improver usedin the solution appears to be dictated by solubility factors, economicconsiderations and any adverse effect that excess amounts of theadhesion improver might have on the coated surface. There has been someevidence that excess amounts of the adhesion improver can tend to affectcorrosion resistant properties adversely. In the case of starchphosphate, this has occured with aqueous solutions containing in excessof about 0.3 wt. percent starch phosphate. It is recommended thatsolutions comprising about 0.02 to about 0.2 percent by weight of theadhesion improver be used. However, it should be understood that greateror somewhat smaller amounts can be used in the solution and/or thecontact time between solution and the surface varied in order to achievethe desired result. Suffice it to say that the amount of adhesionimprover deposited on the surface should be an amount such that thedesired improvements in paint adhesion are realized.

The amount of adhesion improver applied is relatively small compared tothe amount of film-forming resinous coating composition appliedsubsequently. Thus, the weight of the adhesion improver per unit area ofsurface is substantially less than the weight of the polymeric coatingor film formed from the film-forming resinous coating composition. It isnoted that there are applications in which a primer or prime coat offilmforming resinous coating composition is applied prior to theapplication of a top-coat of film-forming resinous coating composition.The weight per unit area and the thickness of a film of adhesionimprover will be substantially less than the weight and thickness of aconventional prime coat, which, for example, may have a thickness assmall as about 0.1 to about 0.3 mils; these are characteristics whichdistinguish a film of adhesion improver from a prime coat, as well as atop-coat, formed from film-forming resinous coating compositions.

TESTS Tests which were used to evaluate the paint adhesive properties ofmetallic surfaces treated according to this invention and the results ofwhich are reported in the Example section hereinbelow are describedhereafter. Olsen Cup Test This test is designed to evaluate paintadhesion under conditions which simulate a forming operation on metal.Generally speaking, the test involves the use of a hand operatedductility testing machine which is used to apply a load gradually to apainted metallic test panel by means of a screw type mechanism havingmounted thereon a ball which forms a cup in the panel.

The machine used in the test is sold by Tinius Olsen Test MachineCompany under the name Hand Operated Ductility Tester. The load isapplied to a test panel and the cup is formed therein by a ballseveneighths inch in diameter and positioned at the end of the screwtype mechanism of the test machine. The test panel, 0.026 inch inthickness, is clamped firmly to the test machine in a position such thatthe ball is forced against the unpainted side of the test panel as thescrew type mechanism is turned. The load is applied until the metalfractures. (In the examples reported herein, this happened when thedepth of the cup formed in the test panels by the ball was about 0.30inch.) Thereafter the load on the test panel is released and the convexpainted side of the cup is checked for cracking and adhesion of thepaint film. (It is noted that cracking of the paint film, without lossof adhesion to the metallic substrate, is caused by lack of cohesion ofthe paint and is not related to the bond between the paint andunderlying surface.) To determine paint adhesion, Scotch Brand No. 600tape is applied to the convex painted surface of the cup and thenremoved. Paint adhesion properties are expressed as the percent of paintlost to the tape in the test area of the panel.

Taper Bend Test This test is designed to evaluate paint adhesion underconditions which simulate roll forming of a painted metal strip. Ametallic test panel is bent by hand to an angle of about with thepainted side of the panel on the outside of the bend. The bend iscompleted by placing the panel in a machinist vise, the jaws of whichhave been positioned such that the space between them is wedge-shaped.As the vise is closed, it bends the panel so that its edges are togetherat one end, and onefourth inch apart at the other end. After the panelis so bent, it is removed from the vise and inspected. A No. 600 ScotchBrand Tape is applied to the paint of the bend and then removed. Paintadhesion properties are measured by the percent of paint lost to thetape and by measuring the distance from the edge of the closedend of thepanel on the bend to the farthest point on the bend from which paint hasbeen removed. Cracking of the paint film absent a loss of adhesion tothe panel is caused by a lack of cohesion of the paint film and not topaint adhesion properties.

EXAMPLES The first group of examples is illustrative of the use .ofstarch phosphate as an adhesion improver in accordance with thisinvention. Various types of metallic surfaces, having thereon pre-paintcoatings, were rinsed with an aqueous solution containing 0.3 wt.percent starch phosphate (Standard grade ARD-l230 sold by AmericanMaize-Products Company). No other ingredients were added to thesolution.

test panels in the solution for 10 seconds;

6 rinsed with an aqueous final rinse solution of hexavalent chromium andreduced chromium for 10 seconds;

7 squeeged and air dried; and

8 painted by draw bar technique with a polyester paint, a white alkydhigh gloss enamel sold by Dupont under the name of Refrigerator WhiteHigh Gloss No. 706B9859l; the paint film (1 mil in thickness) was curedby baking at 300F for 30 minutes.

The panels were then subjected to the tests referred to in Table 1below. Another group of panels was treated in exactly the same wayexcept that after the pre-paint coating was applied, they were rinsedwith tap water instead of the aforementioned aqueous solution of starchphosphate. For comparative purposes, these panels were tested also andtheir test results are set forth in Table 1 below. H

TABLE 1 The striking improvements provided by the invention are evidentfrom the data reported in Table i above. It is noted that theimprovements in the paint adhesion properties of the metallic panelswere achieved with little or no sacrifice in the corrosion resistantproperties. It is noted also that another group of test panels wastreated according to the same procedure as the test panels of Table 1,except that they were painted with an acrylic paint instead of an alkydpaint, and little or no improvement in the paint adhesion properties ofthe acrylic paint was obtained; in fact, in some cases, the paintadhesion properties were affected adversely.

Examples 1-5 of Table 1 illustrate also that the adhesion improvers ofthis invention can be applied to the pre-paint coating and thereafterrinsed with a final rinse solution such as the hexavalent/reducedchromium rinse solution used in the examples. Contrary to what might beexpected, the starch phosphate was not washed off the panels by therinse solution. The test panels were viewed under a microscope after thehexavalent/reduced chromium final rinse was used and before the alkydpaint was applied and particles of starch phosphate were visible.

The next group of examples illustrates the application of an adhesionimprover of this invention to a metallic surface having thereon apre-paint coating by applying thereto the adhesion improver from a finalrinse solution (in contrast to its application prior to a final rinsetreatment as in Examples l-5). A set of hot dipped galvanized panels wasprocessed as follows:

I cleaned with an alkaline cleaning solution;

2 rinsed with tap water;

3 rinsed with an activator;

4 coated with a zinc phosphate coating formed from an aqueous solutioncontaining zinc oxide, phosphoric acid and nitric acid;

5 rinsed with tap water;

6 rinsed with a final rinse solution containing a starch phosphate andhexavalent/reduced chromium by immersing the phosphate coated panel forl0 seconds into a bath of the rinse solution, the temperature of whichwas 120F; and

7 squeeged and air dried; and

8 painted by draw bar technique with a polyester paint, a white alkydhigh gloss enamel sold by Dupont ass st. tbs arss .R t ssr tetitlcsttish (Elastic:

Test results Adhesion S arch Salt spray phos- Taper bend ASTM B-117phate Olsen cup in 32nd Pro-paint coating rinse percent 6 In. Percent 9inch Iron phosphate No 45 2 28 l ....do Yes 0 0 0 3 Zinc phosphate =2No. 40 2 20 3 do Yes 6 l 3 5 Gal hromate N 50 2% 40 2 4 Galv .do Yes...0 O 0 2 Alum... Chromate/fiuoride N 20 1% l0 0 c Alum do Y 0 0 0 0Alum... Chromate/phosphate 5 No 40 1% 15 0 Alum do Yes. 0 0 0 0 l Theiron phosphate coating was formed from an aqueous solution containingphosphoric acid, soad ash and Sodium chlorate.

2 The zinc phosphate coating was formed from an aqueous solutioncontaining zinc oxide,

nickelous oxide and sodium chlorate.

phosphoric acid,

3 The chromatic coating was formed from an aqueous solution containingchromic acid, zinc oxide, hydrochloric acid, and fluosilicic acid.

4 The chromate/fluoride coating was formed from an aqueous solutioncontaining hydrofluoric acid, chromic acid and potassium ierricyanidc.

5 The chronirtte/phosphate coating was formed from an aqueous solutioncontaining a phosphoric acid, chromic acid, hydrofluoric acid, andhydrogen peroxide.

5 Percent of test area incurring paint loss.

7 Distance in inches from edges of closed end of panel to farthest pointon bend at which there was paint loss.

5 Percent of area incurring paint loss.

9 All test panels except the aluminum test panels were subjected to thesalt spray test. for 240 hrs; the aluminum panels were subjected to thetest for 1,000 hrs. The test results represent the average width offailure from the scribe.

706-B-9859l; the paint film (1 mil in thickness) was cured by baking at300F for 30 minutes.

With respect to step (6) above, the starch phosphate was Standard GradeARD-l230 as identified above. The final rinse solution was prepared froman aqueous concentrate which was prepared by dissolving about 42.1 partsby wt. of chromic acid in about 50.5 parts by wt. of water andthereafter adding about 7.4 parts by wt. of formaldehyde (37 percent).The solution was stirred for about 45 minutes until dissolved gasesescaped and thereafter cooled. Water was added to adjust the specificgravity of the concentrate to 1.426 at 60F. The concentrate was dilutedwith an amount of water such that the concentrate comprised about 0.2percent by wt. of the final rinse solution and suitable amounts ofstarch phosphate were added to separate portions of this rinse solutionto give the concentrations set forth in Table 2 below.

The test panels were subjected to the tests referred to in Table 2below. For the purposes of comparison, another set of test panels wastreated in exactly the same way except that the final rinse solutioncontained no starch phosphates. The test results of these panels are setforth also in Table 2.

TABLE 2 Ex. Amt. of Starch Test Results No. Phosphate in Cr AdhesionAqueous Rinse Olsen Taper Bend Salt Spray Solution, by Cup in. ASTM 8-117, in

Weight 32nd inch 6A l4 1 5 l 6 0.01 0 0 O 7 005 u u u u 8 010 u u 9 0.153 10 0.25 2 11 0.35 3

as set forth in Table 1 Some general comments concerning the presentinvention can be made on the basis of the test results reported in Table2 above. From Examples 6-1 1, it can be seen that excellent improvementsin paint adhesion can be obtained with the use of, but relatively smallamounts, of starch phosphate in the final rinse solution. The percent ofstarch phosphate used in Example 6 corresponds to only about 0.10 gramper liter of final rinse solution. It is notedalso that the test resultsof Table 2 make it evident that the improvements in paint adhesion areobtained with little or no loss in the corrosion resist ant propertiesof the coated metallic surfaces.

Aqueous final rinse solutions of the type utilized in the Examples 6-11were prepared and then allowed to stand for extended periods of time forthe purpose of checking their stability. The rinse solutions were stableand effective in use after a 6 month period at which time the stabilitytest was discontinued. It was noticed during the periodic checking ofthe final rinse solutions that there was some decrease in the hexavalentchromium concentration. However, this appeared to have no adverse affecton the effectiveness of the rinse solut se .7 t

In the development of this invention, it was found that the addition ofstarch phosphate to an aqueous concentrate of the type utilized toprepare the dilute final rinse solutions of Examples 6-ll resulted inthe formation of a rigid gel; however, upon dilution with water, theresulting final rinse solution was used effectively. On the other hand,when the aqueous concentrate was allowed to stand for about an hourafter preparation, the gel became watery; and a rinse solution made fromthe aged watery concentrate was not effective. Thus, it is recommendedthat when preparing final rinse solutions of the type reported in Table2 above, the starch phosphate be added to the dilute aqueous final rinsesolution and not the concentrate.

The next group of examples is illustrative of the use of some differentpolyester-type adhesion improvers within the scope of this invention. Aset of hot-dipped galvanized steel test panels was coated with a zincephosphate coating solution containing zinc oxide, phosphoric acid andnitric acid. The zinc phosphate coating weighed about 250 mg. /ft. Thecoated panels were rinsed first with tap water and thereafter with thechromium-containing final rinse solution used in Examples 6-1 1 above,but containing 0.2 wt. percent of the polyester prepared from the acidand alcohol reactants set forth in Table 3 below instead of starchphosphate. The test panels were immersed for 10 seconds in this finalrinse solution which was heated to a temperature of F. After beingwithdrawn, they were squeeged and allowed to air dry. The panels werethen painted in the same way and with the same paint as used in Examplesll l. A

The following procedure was utilized to prepare the polyesters from theacid and alcohol reactants set forth in Table 3 below. For thosereactants for which a molecule weight was known (for all butpolymethylvinylether maleic acid, polyvinyl alcohol and hydroxyethylcellulose), equimolar quantities of the reactants were mixed together.When the molecular weight of a reactant was unknown, the reactants werecombined in equal parts by weight. When both reactants were solids, theywere dissolved and reacted in aqueous solution; otherwise the liquidreactant or reactants formed the liquid reaction mixture. The reactionmixture was heated to C at which temperature a large portion of thewater by-product had evaporated. The removal of water permitted thepolyester to increase in molecular weight. The reaction was allowed tocontinue until the reaction mixture thickened and increased in viscosityto the extent that it had a consistency similar to molasses. Thereafterthe reaction mixture was allowed to cool and 0.2 percent by weight ofthe polyester was added to the hexavalent/reduced chromium final rinsesolution referred to above. Upon adding the polyesters to the finalrinse solutions, they dissolved therein.

Due to the great number of tests that were performed in assembling thedata reported in Table 3 below, there were used for comparative purposesa number of control panels, that is, panels which were treated in thesame way as described above except that they were rinsed with ahexavalent/reduced chromium rinse solution which contained no polyester.Each set of test panels that was processed was compared with a controlpanel for that particular set of panels. The results reported in Table 3are ratings which represent an increase or decrease in the quality ofthe test panels over the control panels which were assigned a rating of5. A rating of 10 represents perfect performance.

i7 id TABLE 3 Polyester forming reactants Test results Adhesion Salkspray A... as... :33 are 25.1%?

Polyrnetlrylvinyl ether maleic acid Phosphoric acid Oxalio acid .dCitric acid Glycerol.

.... ..do Ethylene glycol-- Maleic acid... Hydroxyethyl cellulose" 1Furnario acitL. ..do Azelaic acid... do

Fumaric acid Glycerol. Polymethylvinyl ether maleic acid do d Ethyleneglycol Hydroxyethyl cellulose" Glycerol Ethylene glycol. eerol Ethyleneglycoldo...

Resoreinol Ethylene glyc Hydroquinone Excluding the inorganic acids,each of the reactants in Table 3 is either a polycarboxylic acid or apolyhyd- Polyivinyl alcohol- From Table 4 above, it should beappreciated that each of the polyamides improved paint adhesion mark-119 alcohol- Included among the examples are dical'box edly. Withrespect to the last three examples the overall ylic acids, tricarboyxlicacids and diand tri-hydric alcohols; compounds with greater numbers ofhydroxyl and carboxyl groups are illustrated by the use of polyvinylalcohol and polymethylvinylether maleic acid. It is noted thatExamples12-16 show that polyesters prepared from the long chain polyvinylalcohol reactant give top performance; likewise for the polyesterprepared from the tri-carboxyl and tri-hydroxyl reactants of Example 17and the tricarboxyl and di-hydroxyl re actants of Example 18. Again itis noted that the improvement in paint adhesion is obtained with littleor no sacrifice in the corrosion resistant properties; indeed in somecases a slight improvement in the corrosion resistant properties wasrealized.

The next group of examples illustrates the use of adhesion improvers ofthe polyamide type. The polyarn- I ides that were used were preparedfrom the carboxylic acid and the diamine or the hydroxy-amine reactantsset forth in Table 4 below. (It is noted that the polyamides preparedfrom the hydroxy-amine reactants contained both ester and amidelinkages.) The polyamides were prepared from equimolar quantities of thereactants by simple addition of one to the other. The exothermicreaction was terminated by cooling the reaction mixture when atemperature of 140C had been reached and when the mixture had thickenedand increased in viscosity to the extent that it had a consistencysimilar to molasses. The effectiveness of the polyamides as an adhesionimprover was determined by treating and evaluating a group of testpanels in the same way as those of the examples of Table 3, except thatthe phosphate coatings applied were rinsed with a hexavalcnt/reducedchromium final rinse solution that contained 0.2 percent by weight ofthe polyamides.

43 Maleic acid..- Ethanolarnine.. 9.0

performance was somewhat offset as a result of a reduction in thecorrosion resistant properties.

In another series of tests, there were used two different film-formingresinous coating compositions that contained unsaturated polyestersprepared from unsaturated dicarboxylic acids and diols. These coatingcompositions, which were paints gray and green in color, contained alsoa vinyl monomeric crosslinking agent. Improvements in paint adhesionwere attained when zinc phosphate coated metallic panels were rinsed,prior to painting, with an aqueous final rinse solution that containedstarch phosphate. Thus, this series of tests. exemplified the use of apolyester-containing paint, but one not generally classified as an alkydpaint the paint used in the previous examples.

The examples reported hereinabove illustrate the very great extent towhich this invention provides improvements in the adhesion of acontinuous polymeric film or coating to a metallic substrate coated witha pre-paint coating. Overall, it can be said that these improvements canbe attained in a relatively uncomplicated, economic and practical way.

We claim:

1. A method for coating a metallic surface compris- 0 ing:

A applying to said metallic surface a pre-paint coating-forming solutionwhich forms on said surface a pre-paint conversion coating;

B applying to said pre-paint conversion coating while wet a polyester orpolyamide adhesion improver in adhesion improving amounts by rinsingsaid wet pre-paint conversion coating with a liquid solution containingsaid polyester or polyamide adhesion improver;

C applying to the thus treated coating a film-forming resinous coatingcomposition containing a polyester resin; and

D chemically linking said polyester resin to said adhesion irnproverthrough a multiplicity of bonds by reacting said polyester resin withsaid adhesion improver;

thereby improving the adhesion between said pre-paint conversion coatingand the coating formed from said resinous composition.

2. The method according to claim 1 wherein starch phosphate is appliedto said wet pre-paint conversion coating.

3. The method according to claim 1 wherein said wet pre-paint conversioncoating is rinsed with an aqueous solution containing said polyester orsaid polyamide.

4. The method according to claim 3 wherein said aqueous solution is afinal rinse solution.

5. A method for coating a metallic surface compris- A applying to ametallic surface a pre-paint coatingforming solution which forms on saidsurface a prepaint conversion coating;

B applying to said pre-paint conversion coating while wet a watersoluble or colloidally dispersible polyester or polyamide adhesionimprover, wherein said adhesion improver is applied from a water rinsewhich is applied to said pre-paint conversion coating to removetherefrom excess or spent prepaint coating solution or from a finalrinse solution after said pre-paint conversion coating has been waterrinsed to remove therefrom excess or spent pre-paint coating solution;and

C thereafter applying an alkyd paint to the pre-paint conversion coatinghaving thereon said adhesion improver;

wherein the amount of adhesion improver applied is sufficient to improvethe bond between the paint film formed from said alkyd paint and saidpre-paint conversion coating.

6. The method according to claim 5 wherein said adhesion improver isstarch phosphate and wherein it is applied to said pre-paint conversioncoating by rinsing it with an aqueous solution containing said starchphosphate.

7. A method for coating a metallic surface compris- A applying apre-paint conversion coating to said metallic surface;

B applying an aqueous solution containing starch phosphate to saidpre-paint conversion coating; and

C applying an alkyd paint to said pre-paint conversion coating havingthereon starch phosphate;

wherein the amount of starch phosphate-on said prepaint conversioncoating is at least sufficient to improve the bond between saidconversion coating and the paint film formed from said alkyd paint.

8. A method according to claim 7 wherein said prepaint conversioncoating is a zinc phosphate coating.

9. A metallic surface coated according to the method of claim 1.

10. A metallic surface coated according to the method of claim 5.

11. A metallic surface coated according to the method of claim 7.

} "UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,77,732 I Dated Dgggmbg; 4 1913 Inventor-(s) Thomas J. Kiefer and MasamichiNagatani It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 13, Table l, the alignment of the columns headed "Examplenumber'- and "Metal" should be as follows:

Example Metal number lA Steel 1 do 2A do 2. do 3A Galv 3 Galv 4A Alum 4Alum 5A do Column 13, Table 1, under heading- "Pre-paint coating", "Ironphosphate", should read Iron phosphate( Column 13, first line offootnote (1) "soad should read --soda--.

Column 15, Table 2, heading of first column should read--EX.

No.-- and headingv of second column should read -Amt. of StarchPhosphate in Cr- Aqueous Rinse Solution, by Weight--.

igned and sealed this 2nd day of July 1974.

(SEAL) Attest: v

EDWARD M. FLETCHERJR} C.MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PC4050 (wsg) uscoMM-oc scan-Pee I U.S. GOVERNMENT PRINTINGOFFICE 1 I969 0-366-334.

2. The method according to claim 1 wherein starch phosphate is appliedto said wet pre-paint conversion coating.
 3. The method according toclaim 1 wherein said wet pre-paint conversion coating is rinsed with anaqueous solution containing said polyester or said polyamide.
 4. Themethod according to claim 3 wherein said aqueous solution is a finalrinse solution.
 5. A method for coating a metallic surface comprising: Aapplying to a metallic surface a pre-paint coating-forming solutionwhich forms on said surface a prepaint conversion coating; B applying tosaid pre-paint conversion coating while wet a water soluble orcolloidally dispersible polyester or polyamide adhesion improver,wherein said adhesion improver is applied from a water rinse which isapplied to said pre-paint conversion coating to remove therefrom excessor spent pre-paint coating solution or from a final rinse solution aftersaid pre-paint conversion coating has been water rinsed to removetherefrom excess or spent pre-paint coating solution; and C thereafterapplying an alkyd paint to the pre-paint conversion coating havingthereon said adhesion improver; wherein the amount of adhesion improverapplied is sufficient to improve the bond between the paint film formedfrom said alkyd paint and said pre-paint conversion coating.
 6. Themethod according to claim 5 wherein said adhesion improver is starchphosphate and wherein it is applied to said pre-paint conversion coatingby rinsing it with an aqueous solution containing said starch phosphate.7. A method for coating a metallic surface comprising: A applying apre-paint conversion coating to said metallic surface; B applying anaqueous solution containing starch phosphate to said pre-paintconversion coating; and C applying an alkyd paint to said pre-paintconversion coating having thereon starch phosphate; wherein the amountof starch phosphate on said pre-paint conversion coating is at leastsufficient to improve the bond between said conversion coating and thepaint film formed from said alkyd paint.
 8. A method according to claim7 wherein said pre-paint conversion coating is a zinc phosphate coating.9. A metallic surface coated according to the method of claim
 1. 10. Ametallic surface coated according to the method of claim
 5. 11. Ametallic surface coated according to the method of claim 7.